Electrical apparatus for use in explosive atmospheres, especially fire-damp environments, e.g. in mine galleries and shafts, requires special construction for safety and the prevention of explosions upon operation of the unit, e.g. a spark-producing switch, contained in the apparatus housing. The goal of the special measures which are used, of course, is to prevent any possible spark generated by the unit from igniting a possibly explosive mixture of gases surrounding the apparatus. However, while the principles of the invention will be described using the requirements for application of such explosive environments as a starting point, it should be emphasized that the use of the apparatus which will be described herein is in no way limited to such explosive environments.
There are various ways of protecting electrical apparatus so as to prevent explosions and for each of the protection techniques, there is a corresponding type of apparatus. Perhaps the technique most generally applied is that of pressure-tight or hermetic encapsulation, whereby the electrical device or unit is enclosed in a pressure-sustaining housing so that any explosion developing within the housing remains contained and is not propagated to the surroundings. Other techniques include plate-protective encapsulation, oil encapsulation and external air introduction (flushing).
In recent years over-pressure encapsulation has been developed. In this technique, the explosion-sensitive unit has been enclosed in a housing which is maintained at a superambient pressure so that the incursion of gas contributing to air constituting an explosive mixture is precluded. The gas which maintains the superatmospheric pressure is usually air or an inert gas, for example, nitgrogen.
The over-pressure encapsulation has the advantage over pressure-retentive encapsulation that the superatmospheric pressure of the gas within the housing excludes the formation of an explosive gas mixture therein so that the danger of an explosion within the housing which must be contained thereby is reduced or eliminated. Since an explosive gas mixture is thereby prevented from coming into contact with the possible spark within the housing in the first place, the reliability and safety of the antiexplosion measure is clearly enhanced by comparison with conventional pressure-retentive techniques in which explosions may occur within the housing.
With time, generally as a result of the fact that seals may not always be perfect, the superambient pressure of the gas within the housing may drop and, consequently, it has been the practice heretofore in over-pressure encapsulation techniques to provide the housing with an additional opening and fitting through which the pressurizing gas is pumped from time to time to maintain the superambient pressure. The fitting is customarily provided with an automatically closing valve, for example, a check valve, blocking reverse flow through this fitting.
It is also common in such systems to provide a pressure sensor within the housing to respond to the gas pressure therein and to provide means which can be monitored externally of the housing, either in the vicinity thereof or at a remote location, to enable the pressurization status of the housing to be continuously or periodically evaluated and to permit automatically or with operator intervention the pumping of additional gas under pressure into the housing to compensate for any pressure drop.
The foregoing discussion makes clear that up to now the housing used for over-pressure encapsulation has had to be of special construction in no small measure because of the need for the additional pressurizing-gas opening and fitting. Naturally this makes it practically impossible to utilize the over-pressure technique on existing pressure-retentive housings which are not provided with such openings and fittings and operated in the conventional manner.